1. Field of the Invention
This invention pertains to light-emitting diodes and, more specifically, to light-emitting diodes having a resonant-cavity design.
2. Description of Related Art
A resonant-cavity light-emitting diode has an advantage over a conventional light-emitting diode because of its higher efficiency. This means that more optical power can be provided by the device at a certain operating current. Another advantage is that a vertical light emission is naturally achieved. A resonant-cavity light-emitting diode also has advantages over a semiconductor laser, in particular over a vertical-cavity surface emitting laser, because its emission spectrum may have a spectral width of 10 nm or more, being rather smooth (without lasing modes).
A long-wavelength resonant-cavity light-emitting diode made on a GaAs substrate may have advantages over long-wavelength resonant-cavity light-emitting diodes made on substrates of other types because of the cheapness of GaAs substrates, their availability with large diameter (e.g. 6 inches), their high quality, and ease of formation of highly-reflective distributed Bragg reflectors in an AlGaAs material system. A disadvantage of current long-wavelength resonant-cavity light-emitting diodes made on a GaAs substrate is a lack of a light-emitting active region, having a sufficient structural and optical quality, being capable of emitting at sufficiently long wavelengths.
InGaAsN quantum wells and (In,Ga)As quantum dots have been recently proposed as an active region of long-wavelength light-emitting devices fabricated on GaAs substrates. An InGaAsN quantum well has a disadvantage of typically low radiative recombination efficiency, which makes it difficult to fabricate a device with sufficiently low operating current. (In,Ga)As quantum dots may be free of this disadvantage.
A method for a resonant-cavity light-emitting diode forms a resonant cavity including a light-emitting active region on a bottom-side distributed Bragg reflector having high reflectivity within a certain stop-band. Because of the joint effect of the distributed Bragg reflector and the resonant cavity, intensity of the light which is emitted from the surface of the device is enhanced while the emission spectrum is modified. An output spectrum of a resonant-cavity light-emitting diode may include several spectral maxima with spectral positions that correspond to spectral minima in a reflectance spectrum of the resonant-cavity light-emitting diode.
It is usually preferred that an output spectrum of a long wavelength resonant-cavity light-emitting diode has a single spectral maximum or at least the main maximum dominates well over maxima, other than the main maximum. However, a quantum dot active region is usually characterized by a relatively broad spectrum of emission due in particular to luminescence of excited state(s). This excited-state luminescence may be stronger than the ground-state luminescence if a current density is sufficiently high. The excited-state luminescence may result in appearance of additional maximum or maxima in the output spectrum of the quantum-dot resonant-cavity light-emitting diode. Moreover, intensity of these maxima may be high in comparison to an intensity of the main maximum of the output spectrum. Such a behavior significantly restricts possible device applications of quantum-dot resonant-cavity light-emitting diodes.
Thus, there is a need in the art for a resonant-cavity light emitting diode made on a GaAs substrate with self-organized quantum dots as the light-emitting active region, the diode being capable of emitting in a range from approximately 1.15 to 1.35 μm, having a sufficiently high optical power and sufficiently low operating current, and having intensity of maxima, other than the main maximum of the emission spectrum, not higher than 1% of an intensity of the main maximum.